Dolphins have names for each other and call for each other just like humans, say scientists.

They use specific signature whistles for family members and social companions - the only animal species apart from ourselves known to do this.

A study of wild bottlenose dolphins off the east coast of Scotland found they responded to their 'own' whistles by calling back.

Dolphins have names for each other and for call each other just like humans. They use signature whistles for family members and social companions - the only animal species apart from ourselves known to do this

The findings suggest dolphins use signature whistles as labels to address or contact individuals of the same species they meet at sea.

And the use of identity signals mirrors the human convention of naming people, said biologists Dr Stephanie King and Dr Vincent Janik of St Andrews University in Fife.

The researchers played back a synthetic recording of a dolphin's own signature whistle, the whistles of familiar dolphins from the same population and unfamiliar ones from a different group.

Monitoring responses from a small boat the dolphins reacted to their own signature whistles but did not respond to others, reports Proceedings of the National Academy of Sciences.

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Dolphin whistles - high-pitched clicking and squeaking sounds - can be detected up to 20 km away (12.4 miles) depending on water depth and frequency.

The researchers said naming is one of the defining features of human language and dolphins have their own unique identity - the signature whistle.

They said: 'Here we show wild bottlenose dolphins respond to hearing a copy of their own signature whistle by calling back. Animals did not respond to whistles that were not their own signature.

'Bottlenose dolphins therefore appear to be unique as non-human mammals to use learned signals as individually specific labels for different social companions in their own natural communication system.'

Dolphin whistles - high-pitched clicking and squeaking sounds - can be detected up to 20 km away (12.4 miles) depending on water depth and frequency. The researchers said naming is one of the defining features of human language and dolphins have their own unique identity - the signature whistle

The dolphin's constantly changing social group - coupled with their poor vision underwater - is believed to have led to the development of the phenomenon.

Dr Janik said: 'We followed the dolphins in small boats and recorded their calls with underwater microphones on ropes and chains.

'The whistles are very different with a variety of modulation patterns going up or down - almost like a tune. Even our closest relatives the great apes can't do this.

'It's interesting to find another mammal like humans with complex intelligence and social structure communicating in this way.

'People see dolphins in crystal clear waters in TV documentaries and don't realise they spend 80 percent of their time in murky oceans with little light.

'They can't use smell to locate partners or relatives and they are not territorial - the open ocean is their home.

'So they have developed this sophisticated sound system to keep in touch otherwise they could get lost from each other for good.'

The researchers said it's clear signature whistles have meaning in they are labels for individuals and may be induced by an intention to contact a specific individual.

They said: 'Such a representational use of learned identity labels represents an interesting parallel to humans and the apparent necessity for these vocal labels in maintaining group cohesion may lie at the root of the evolution of complex communication and cognition systems.'